1. Field of the Invention
The present invention relates to a liquid etchant composition for use in fabrication of a thin film resistor element in an electronic circuit, comprising chromium, silicon and oxygen (hereinafter referred to as "Cr.SiO.sub.2 "), more particularly to a liquid etchant composition for use in etching the Cr.SiO.sub.2 resistor material having an aluminum layer applied as conductor thereon, with an undercut-free shape at the pattern edge of the aluminum layer and a process for etching the Cr.SiO.sub.2 resistor material in an easy and controllable manner. Furthermore, the liquid etchant composition of the present invention is more suitable and useful when the resistor film is formed on an organic insulating film such as a polyimide film.
2. Description of Related Art
Electronic integrated circuits used in a computer are terminated with resistors having the same resistance as that of a transmission line impedance for the purpose of obtaining impedance matching between the circuit elements and the transmission line to inhibit reflection noise generated in transmitting high-speed digital signals. That is, a so-called impedance matching by signal termination resistors has been adopted in the integrated circuit application systems. Therefore, termination resistors must be arranged in or about a package at the same time when the integrated circuits are to be packaged on a board, thereby terminating the transmission line.
Hitherto, resistors for the impedance matching have been externally mounted as independent parts on printed circuit boards or multichip module boards. However, the external mounting of the independent parts is improper for miniaturization of the integrated circuits and improvement of packaging density.
Built-in resistor elements contained in packaging boards could greatly improve the packaging density of the integrated circuits. In a method therefor which has been proposed, a thin film resistor is formed on the surface of a certain layer of a packaging board having a multilayered structure, and the thin film is then fabricated by photoetching into an element shape to have a desired resistance. According to this method, the packaging density cannot only be improved but also a high resistance-accuracy termination resistor element can be formed, and further the length of wirings can be shortened so that the signal-propagating speed is greatly improved.
As a resistor material useful for built-in resistor element in the packaging board has been developed. The material is a composite of SiO.sub.2 and Cr metal which is deposited as thin film by RF magnetron sputtering (see H. Matino and T. Ushida, "Effect of Substrate Bias on Properties of RF-Sputtered Cr-SiO.sub.2 Films", IBM J. RES. DEVELOP., November 1977, pp. 576-579). The liquid etchant composition for this resistor material has been demanded.
In order to secure an ohmic contact between the terminating resistor element and the upper wiring, it is desired that the resistor film and the wiring film are formed in a continuous deposition process. On the other hand, this method has a problem that an adequate pattern-forming accuracy is hardly obtained when the wiring film is thick. It is known that this problem has been overcome to secure formation of the high-accuracy resistor pattern by providing a thin wiring film and then forming a two-layer film pattern of the resistor material and the wiring material in a continuous manner by a photoetching process and thereafter forming a film of the wiring material which has a desired thickness to form a termination resistor pattern.
Moreover, large-scaled computers or supercomputers have a tendency to use a thin film multilayered wiring board using an organic insulating material having a low dielectric constant, such as polyimide, in a packaging board such as a microchip carrier or multichip module board for the purpose of shortening the package-originated signal propagation delay. Now, the abovementioned termination resistor element has been necessitated to be formed on the organic insulating film, for example, polyimide.
As prior etching compositions for etching the Cr.SiO.sub.2 resistor material film is known a liquid mixture (or mix) of nitric acid and hydrofluoric acid as disclosed in Japanese Patent KOKAI (Laid-Open) No. 60-83301. However, this liquid mixture has two problems when used for etching a laminate comprising the Cr.SiO.sub.2 film and an aluminum layer on the film (hereinafter referred to as "Al/Cr.SiO.sub.2 laminate"): the first problem is that the resistor film is undercut so that the aluminum layer forms an eave (see FIG. 1D attached hereto) and the second problem is that the etch residues are easily produced when the Cr.SiO.sub.2 film is etched by the prior etchant. In the first problem, an aluminum electrode film in cracked at the Cr.SiO.sub.2 pattern edge when applied in the subsequent electrode deposition step onto the Al/Cr.SiO.sub.2 laminate. The cracks propagate from the eave region through, whole Al film to finally reach the top Al surface. FIGS. 2A to 2D show an etching process showing schematical cross-sectional views of a sample at each step. In these figures, reference number 1 denotes a patterned photoresist, 2 an aluminum layer, 3 a Cr.SiO.sub.2 layer, 4 a polyimide layer, 5 a ceramic or glass substrate and 6 etch residues. FIG. 2A shows the sample having a patterned photoresist film and unetched Al/Cr.SiO.sub.2 laminate. FIG. 2B shows the sample having the etched Al/Cr.SiO.sub.2 laminate. FIG. 2C shows the sample having the photoresist removed. FIG. 2D shows the sample having a thick aluminum electrode formed thereon and illustrates that microcracks 8 are produced in the Al electrode during or after the deposition of the electrode when the resistor film has undercut. It shows that the microcrack occurs because of insufficient Al coverage caused by the undercut of the resistor film. These microcracks may be a cause of the open failure of the aluminum electrode, which deteriorate the reliability of integrated circuits. If the aluminum eave is large enough, it falls down and therefore the Al electrode is badly deformed. In the second problem, the residues are more easily produced particularly when the Cr.SiO.sub.2 film is deposited on the organic insulating film of polyimide or the like. Therefore, there arise problems caused by the etch residues, that is, a bad effect on pattern-wise etching of underlying polyimide or adhesion damage between the polyimide and a next thin film on that.